Geof,
I think you have correctly identified the reason your cowl box works to lower coolant temperature.
I presume you have the standard radiator.
My 2008 4/4 1.8 had to have a replacement radiator when the plastic inlet top pipe fractured. The new one is all aluminium by Mulfab and longer top to bottom. The lower edge comes about 5 inches lower than the original. I have taken temperature readings off the radiator and discovered that the lower 20% remains cold with normal driving. Spirited driving heats it more as you would expect but the electric fan rarely comes into operation. No cowl is fitted.
